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Cash, Cash Equivalents, Restricted Cash and Marketable Securities - Additional Information (Detail) - USD ($) | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017
Investments Debt And Equity Securities [Abstract] |  |
Realized gains or losses from the sale of marketable securities | $ 0 | $ 0

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- Definition Amount of realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI). + References No definition available. + Details
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